High-power fiber lasers typically use Large-Mode-Area (LMA) optical fiber, whose core size is made relatively large in order to decrease the light intensity in the fiber core at a given optical power to avoid fiber damage threshold and performance degradation due to optical non-linearities in the fiber core. However, typical LMA fibers that are currently used in commercial products are multimode fibers, which may degrade the quality of a light beam at the fiber output.
Indeed, the quality of the light beam from an optical fiber output, which defines for example how well the beam may be focused using conventional optical elements, depends on a modal composition of the beam. One conventional way to describe the beam quality is by means of a so-called M2 factor, also known as the M2 factor, which relates how much the intensity profile of the beam deviates from an ideal intensity profile of diffraction-limited Gaussian beam. For a theoretical TEM00 mode, which is easiest to focus, M2=1; for a real-life light beam, M2>1. The fundamental mode of a typical multimode optical fiber, such as the LMA fiber used in the high-power fiber lasers, carries the best beam quality compared to all other modes of the fiber, with M2-factor around ˜1.1. The higher-order-modes (HOM) of a typical multimode fiber produce worse beam quality with larger value of M2. Typical values of the quality factor M2 for the four lowest-order HOM of an LMA fiber may for example be as follows: from M2˜1.1 to M2˜3.3 for mode LP11, M2˜3.3 for mode LP21, M2˜3.3 for mode LP02, M2˜4.2 for mode LP31, with even greater M2 values for modes of higher orders. Thus, the beam quality factor for a fiber output beam composed of the fundamental mode and the four next higher order modes may be between M2˜1.1 and M2˜4.2 depending on the exact modal composition.
It may therefore be desirable to have the ability to evaluate the modal composition of the light beam at the fiber output, i.e. a relative contribution of different fiber modes into the fiber output light beam emitted from the fiber. It is also often desirable to minimize the HOM contribution and maximize the contribution of the fundamental mode into the fiber output beam from a multimode fiber.
Furthermore a typical commercial fiber laser system may include a monolithic strand of optical fiber having multiple fiber-to-fiber splicing points where different types of optical fibers are spliced together. Commercially available fiber splicing devices which can splice together two strands of fiber, also known as fiber splicers, may include a built-in transverse imaging system that provides an image of the fiber ends to assist in geometrically aligning two fiber ends for splicing. This imaging-aided alignment procedure works reasonably well for telecom-grade single-cladding single-mode fibers (SMF) with small-size round-shape geometries that are manufactured using standardized procedures, wherein variations in fiber geometry, such as core/cladding size, shape, and centration, are relatively small. For example, splicing two strands of a standard SMF fiber of the same type, such as “SMF-28e”, “HI1060”, and “PM980”, may produce splicing points with less than 0.1 dB power loss and little, if any, beam distortion at the splice point.
However, fiber alignment that relies on a built-in imaging system may be less reliable for splicing a multimode fiber, such as an LMA fiber of the type used in high power fiber laser systems, with another LMA fiber of a different design or with a smaller-core fiber. Firstly, LMA fibers are often customized and may have significant lot-to-lot variation in size, shape, and centration. Furthermore, a fiber laser typically includes splices between different fiber components, such as gain fibers, pump combiners, fiber gratings, isolator pigtails, etc., which may have different core/cladding geometries and may be produced by different manufactures and/or to different standards, resulting in different cladding/core sizes, shapes and centrations as well. Furthermore, in order to increase optical gain, the cladding of active-core fibers that are used as gain elements usually has a non-round, e.g. octagonal, shape, which may further complicate fiber alignment using conventional transverse imaging systems.
For these reasons, the fiber splicing in high power fiber lasers may benefit from a feedback system to guide the splicing alignment process. In one example wherein the fiber components can be designed to have identical cores, the feedback system for splicing alignment may be based on optimizing core-to-core power transmission between the two fibers, or core-to-clad power ratio, i.e. the ratio between the percentage of light in the core of a second fiber versus the percentage of light in its cladding. This is illustrated in FIGS. 1a and 1b, with FIG. 1b in particular showing that a misalignment of proximate ends of fibers 1 and 2 having identical-size fiber cores 3 would launch light from the core 3 of one of the fibers into the cladding of the other fiber, so that the desired “on-axis” alignment illustrated in FIG. 1a would correspond to the maximum in-core power transmission between the two fibers, or a minimum core-to-clad power ratio at the end of the 2nd fiber. Since the core-clad power ratio has better contrast than in-core power transmission, it may be preferred for use as the external feedback for splicing alignment of identical core-size fibers. However, this method may be less reliable for splicing two fibers having cores of different diameters, as illustrated in FIGS. 2a and 2b. FIG. 2b in particular illustrates by way of example a misalignment between two fibers of different core size, wherein fiber 1 has a core 4 that is smaller in diameters that the core 5 of fiber 2. In this case, an off-axis alignment of the type illustrated in FIG. 2b may not be reliably detected by either the in-core power transmission or the core-to-clad power ratio.
Accordingly, it may be understood that there may be significant problems and shortcomings associated with current solutions and technologies for enabling accurate alignment and splicing of large-core multimode optical fibers, fibers of complex core/cladding structures, and/or of differing core size.